Isotypic surface glycoproteins of trypanosomes

We have identified cloned populations of trypanosomes, derived from different original field isolates and species, which appeared to possess similar variable surface-antigen types when assayed by immune lysis and immunofluorescence tests. Variable surface glycoproteins (VSGs) were isolated from these 'isotypic' trypanosomes. Isolated VSGs had identical apparent mol. wt of 60,000 but different isoelectric points. Radioimmunoassay, using 125I-labelled VSGs and rabbit anti-VSG sera, showed that isotypic VSGs contained not only the cross-reacting determinants found in all other VSGs (Barbet & McGuire, 1978) but also other cross-reacting determinants shared between isotypes. Peptide mapping revealed extensive amino-acid sequence homology between isotype VSGs.